Capacitance type touch panels, which detect the position of an input by an input operation member such as a finger, are categorized into a self capacitance type (one-wire type) and a mutual capacitance type (two-wire type). In the self capacitance type touch panels, detection electrodes having an increase in stray capacitance by approach of the input operation member are detected to detect the input operation position from the arrangement position of the detection electrodes. In the mutual capacitance type touch panels, an alternating current detection signal at a predetermined voltage level is output to drive electrodes, and detection electrodes at which detection voltages of the detection signal are decreased by approach of the input operation member are detected to detect the input operation position from the arrangement position of the detection electrodes. The former type has simpler structure because there is no need to provide the drive electrodes, but the latter mutual capacitance type is adopted in general because the stray capacitance to be detected is at a minute level of 10 to 20 pF of being difficult to detect.
In the mutual capacitance type touch panels, a plurality of drive electrodes to which the detection signal is output and a plurality of detection electrodes for detecting the detection voltages in which the detection signal manifests itself are arranged orthogonally to each other. A voltage variation level of the detection voltage is monitored at each individual intersection point at which the drive electrode and the detection electrode intersect. The input operation position is detected from the position of the intersection points of the detection electrodes that have the voltage variation levels equal to or more than a predetermined threshold value by approach of the input operation member (For example, Patent Literature 1).
Such capacitance type touch panels detect the input operation position from a variation in feeble capacitance around the detection electrodes, and hence are sensitive to noise occurring in a display device disposed in the vicinity of the touch panel and electrostatic noise in surrounding stray capacitance. The noise sometimes causes a false detection of the input operation position.
As a method for improving the detection accuracy of the input operation position by removing the noise, Patent Literature 2 proposes a touch panel in which an AC signal component is extracted as a noise signal from an analog signal detected by a sensor for detecting the input operation position, and the extracted AC signal component is reversed 180° in phase and added to the original analog signal, whereby the input operation position is detected from the analog signal in which the noise signal is canceled.
Also, Patent Literature 3 describes a capacitance type touch panel that is provided with sub sensors for removing noise, in addition to main sensors for detecting the input operation position, in the same input operation surface of the touch panel. In the capacitance type touch panel described in the Patent Literature 3, both of the main sensors and the sub sensors receive various types of noise signals occurring in the vicinity of the input operation surface. The signal received by the sub sensors is subtracted from the signals received by the main sensors by subtracters to remove noise, so that the input operation position is detected from the outputs of the subtracters, which include only signals by an input operation.